Turbocharging an engine allows the engine to provide power similar to that of a larger displacement engine. Thus, turbocharging can extend the operating region of an engine.
Turbochargers function by compressing intake air in a compressor via a turbine operated by exhaust gas flow. Under certain conditions, the flow rate and pressure ratio across the compressor can fluctuate to levels that may result in noise disturbances, and in more severe cases, performance issues and compressor degradation. Such compressor surge may be mitigated by one or more compressor bypass valves (CBVs). The CBVs may recirculate compressed air from the compressor outlet to the compressor inlet, and thus may be arranged in a passage which is coupled to the intake upstream of the compressor and downstream of the compressor in some examples. In some examples, continuous CBVs (CCBVs) may be used, which provide a continuous and continually variable circulation flow from downstream of the compressor to upstream of the compressor. CCBVs may provide boost control and compressor surge avoidance, and may further prevent objectionable audible noise. However, incorporation of such valves can add significant component and operating costs to engine systems.
Engines may also include one or more aspirators may be coupled in an engine system to harness engine airflow for generation of vacuum, for use by various vacuum consumption devices that are actuated using vacuum (e.g., a brake booster). Aspirators (which may alternatively be referred to as ejectors, venturi pumps, jet pumps, and eductors) are passive devices which provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive air flow rate through the aspirator. For example, when incorporated in an engine intake system, aspirators may generate vacuum using energy that would otherwise be lost to throttling, and the generated vacuum may be used in vacuum-powered devices such as brake boosters. While aspirators may generate vacuum at a lower cost and with improved efficiency as compared to electrically-driven or engine-driven vacuum pumps, their use in engine intake systems has traditionally been constrained by both available intake manifold vacuum and maximum throttle bypass flow. Some approaches for addressing this issue involve arranging a valve in series with an aspirator, or incorporating a valve into the structure of an aspirator. Such valves may be referred to as aspirator shut-off valves (ASOVs). An opening amount of the valve is then controlled to control the motive air flow rate through the aspirator, and thereby control an amount of vacuum generated at the aspirator. By controlling the opening amount of the valve, the amount of air flowing through the aspirator and the suction air flow rate can be varied, thereby adjusting vacuum generation as engine operating conditions such as intake manifold pressure change. However, again, adding valves to engine systems which already include various valves serving other purposes (such as CBVs) can add significant component and operating costs.
The inventors herein have recognized that aspirators and corresponding ASOVs may be arranged in an engine system in a configuration which maximizes vacuum generation during boost and non-boost conditions, and which enables the ASOVs to serve as compressor bypass valves controllable to selectably divert flow through each of none, one, and both of the aspirators to regulate boost and/or reduce surge during boost conditions. Accordingly, the technical result achieved by the engine systems described herein includes the use of multiple tap aspirators to serve as compressor bypass valves providing a selectable discrete level of compressor bypass flow while simultaneously generating vacuum for use by various engine vacuum consumers and/or for purging a fuel vapor canister during boost conditions, as well as the use of multiple tap aspirators to provide a selectable discrete level of vacuum generation for use by various engine vacuum consumers and/or for purging a fuel vapor canister during non-boost conditions. For example, a first aspirator comprising a suction tap in a throat, a suction tap in a diverging cone, and a suction tap in a straight tube downstream of the diverging cone may couple an inlet of the intake passage (e.g., downstream of an air filter) with the intake manifold for vacuum generation during non-boost conditions, while a second aspirator comprising a suction tap in a throat, a suction tap in a diverging cone, and a suction tap in a straight tube downstream of the diverging cone may couple a main throttle inlet (e.g., downstream of a charge air cooler) with a compressor inlet (e.g., downstream of an air induction system throttle) for vacuum generation during boost conditions. Intake air may be selectably diverted around the compressor and through the first and second aspirators based on desired vacuum generation during non-boost conditions and based on desired compressor bypass flow during boost conditions. During non-boost conditions, for example, intake air may be diverted through each of none, one, and both of first and second aspirators based on the desired vacuum generation, whereas during boost conditions, intake air may be diverted through each of none, one, and both of the first and second aspirators based on the desired compressor bypass flow. Accordingly, during boost conditions, the first and/or second aspirator may function as a compressor bypass valve. Due to the particular arrangement of the first and second aspirators within the system, the first aspirator operates in reverse flow when acting as a compressor bypass valve (in that motive flow travels from its mixed flow outlet to its motive inlet), whereas the second aspirator operates in forward flow when acting as a compressor bypass valve (in that motive flow travels from its motive inlet to its mixed flow outlet).
While aspirators each having a single tap or suction port may be used, the inventors herein have recognized that a multiple tap aspirator which includes suction taps in the throat, diverging cone, and straight exit tube of the aspirator may advantageously maximize vacuum generation while enabling a high suction flow rate, in that this placement combines the advantages of throat tap aspirators (e.g., high vacuum generation) with the advantages of aspirators with taps arranged downstream of the throat (e.g., high suction flow). Inclusion of a tap in the exit tube (e.g., a straight, unconstricted tube downstream of the aspirator's diverging cone) advantageously enables fast pull-down of a vacuum source, such as a brake booster. Further, the inventors have recognized that such an aspirator may be powered by vacuum rather than compressed air, and that flow losses which often occur in staged aspirators featuring multiple check valves in the suction flow path may be reduced via a configuration wherein only a single check valve in the path between the source of suction flow and each suction tap of the aspirator.
should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.